Accompanying FIG. 1 is a diagram of a conventional three-phase alternator circuit. The circuit comprises, in particular, three star-connected alternator secondary windings EN1, EN2, and EN3, a rectifier bridge P made up of three pairs of diodes, and a regulator R having two phase inputs .phi.1 and .phi.2 receiving two of the three phase voltages from the secondary windings EN1, EN2, and EN3.
Conventionally, the regulator R has a circuit DP for detecting the appearance of a voltage difference between the two phase terminals .phi.1 and .phi.2 when the alternator is caused to rotate, and for triggering regulation of the battery voltage when said rotation has been detected in this way. Such a circuit DP can be used for ensuring that the alternator is self-starting.
Numerous circuits for detecting a voltage difference between the phase terminals of an alternator are already known. By way of example, reference can advantageously be made in this respect to the following patents and patent applications: FR-2 343 255, FR 2 730 359, and FR-2 678 070.
The circuits described in FR-2 343 255 and FR-2 730 359 have the advantage of good detection sensitivity (in particular 50 mV).
However, the inventors have found it to be desirable, particularly when testing regulators during manufacture or prior to mounting, to be able to apply voltage levels to the phase inputs .phi.1 and .phi.2 that are different from the voltage levels that said inputs receive when the alternator is in operation, and in particular to be able to apply to said phase inputs, voltage levels that are highly negative.
In particular, in a patent application filed on the same day as the present application, the inventors propose integrating in a regulator means for detecting such a particular voltage level on at least one phase terminal and for triggering operation of the regulator in an accelerated mode when said voltage level is detected. Under such circumstances, and without any need to provide additional input pins on the regulator, a regulator is provided which can be tested by implementing a series of operations in an accelerated mode for testing the functions of said regulator (time delay on starting, progressive charging, etc.), which would otherwise take place, in normal operation of the regulator, over several seconds.
Unfortunately, the circuits described in FR-2 343 255 and FR-2 730 359 do not enable a voltage difference to be detected between their two inputs when both inputs are taken to voltages that are highly negative.
In the circuit described in FR-2 343 255, the transistors of the current sources become non-conductive when negative voltages are applied to both inputs of the circuit.
In the circuit described in FR-2 730 359, accuracy degrades seriously when the voltages of the inputs of the circuit go beyond alternator feed voltage values.
FR-2 678 070 discloses a detection circuit suitable for operating even when highly negative voltages are applied to said two phase inputs .phi.1 and .phi.2.
Nevertheless, that circuit can respond at best to a detection threshold level of 600 mV, i.e. a level which does not enable self-starting to occur in alternators rotating at low speed.